1. Field of the Invention
The present invention relates to a liquid crystal display device widely incorporated in a liquid crystal panel for OA (Office Automation) apparatus, AV (Audio Vidual) apparatus or the like, and a method for producing the liquid crystal display device.
2. Description of the Related Art
Presently, various display modes have been found for liquid crystal display devices. Particularly, liquid crystal display devices having TN (Twisted Nematic) display modes, STN (Super Twisted Nematic) display modes, or the like, are mass-produced.
Usually, a liquid crystal material used for realizing any one of the above-mentioned modes is used with a chiral agent which adjusts a helical pitch of the liquid crystal material. The helical pitch of the liquid crystal material can be controlled and determined whether to increase or decrease upon a rise in the temperature in accordance with the kind of the chiral agent to be used.
Presently, CN (Cholesteryl nonanoate produced by Chisso Corp.) or S-811 (produced by Merck & Co., Inc.) is generally used as the chiral agent. These chiral agents increase the helical pitch of the liquid crystal material upon rise in temperature. Generally, the helical pitch of the liquid crystal material may be either increased or decreased upon a rise in the temperature as long as a stable orientation, i.e., a stable d/p value of liquid crystal molecules can be achieved at an operating temperature of the liquid crystal display device, namely, a temperature in a range of about 0.degree. C. to about 40.degree. C. The d/p value is defined by a thickness d of a liquid crystal cell and a helical pitch p, of the liquid crystal material.
A technique is described in Japanese Laid-Open Patent Publication No. 7-258641 in which a mixture of two kinds of optically active substances is used where one increases a helical pitch of a liquid crystal material and the other decreases the helical pitch of the liquid crystal material upon a rise in the temperature. Accordingly, a helical pitch of the liquid crystal material hardly changes upon a rise in the temperature.
As described in Japanese Laid-Open Patent Publication No. 7-218915, in the case where a surface of each substrate is rubbed, the twist angle of the liquid crystal molecules is determined by a helical pitch p obtained at a nematic-isotropic phase transition temperature (hereinafter, simply referred to as "T.sub.N-I ") (C.degree.) of a liquid crystal material with a chiral agent added, a cell thickness d and surface controlling force caused by a rubbing treatment. Therefore, the difference between a d/p value obtained at the vicinity of T.sub.N-I and a d/p value obtained at a temperature where the liquid crystal display device is in operation should not be large.
Moreover, Japanese Laid-Open Patent Publication No. 6-265899 describes a method for obtaining an orientation at an interface of each substrate without performing a rubbing treatment and by utilizing a helical pitch of a liquid crystal material. According to this method, a chiral agent is used that decreases the helical pitch of the liquid crystal material upon a rise in temperature so as to restrain stripe domains from generating, thereby preventing deteriorations of a display contrast and a response rate of the display. In this case, liquid crystal molecules are oriented in a twisted manner, the orientation being determined by a d/p value at T.sub.N-I of the liquid crystal material with the chiral agent added. Thus, for example, in order to produce a TN type liquid crystal display device having liquid crystal molecules twisted by 90.degree. between a pair of substrates, the cell gap (the cell thickness) d and the helical pitch p of a liquid crystal cell is adjusted so as to satisfy the relationship, d/p=0.25 at the vicinity of the T.sub.N-I The present inventors have proposed a liquid crystal display device including a display medium having liquid crystal regions microscopically divided by polymer walls (Japanese Laid-Open Patent Publication No. 6-301015). The liquid crystal display device is produced as follows.
First, a pair of substrates, each having transparent electrodes for driving the liquid crystal display device, are provided in an opposing manner such that the transparent electrodes are located inside. Subsequently, a mixture containing at least a liquid crystal material, a chiral agent, a photopolymerizable resin and a photopolymerization initiator is interposed between the opposing pair of substrates. Then, the substrates including the mixture are heated to such a degree that the display medium inside is in an isotropic state. Thereafter, UV light having distribution of weak and strong intensities is radiated to the mixture while maintaining the temperature.
Then, the liquid crystal material and the photopolymerizable resin are phase separated by gradually cooling to room temperature with the photopolymerizable resin placed in regions radiated with light of strong intensity and the liquid crystal material placed in regions radiated with light of weak intensity, thereby producing the liquid crystal device.